This invention relates to an improved mold structure and method of molding for open-cast molding of urethane foam from a mechanically frothed heat curable urethane foam. More particularly, this invention relates to an improved mold structure and a method of molding to insure uniform filling of a mold with a mechanically frothed heat curable urethane foam composition, especially to form annular molded parts.
Mechanically frothed heat curable urethane foams and the method of manufacture thereof are known from U.S. Pat. No. 3,772,224. These foams are usually processed by continuously casting the liquid froth onto a suitable carrier or substrate, doctoring the liquid froth to form a continuous wet foam sheet of uniform thickness, and heat curing the liquid froth to form a solid continuous urethane foam sheet of uniform thickness. The general idea of molding of discrete shapes from heat curable mechanically frothed urethane is also taught in U.S. Pat. No. 3,772,224 and in U.S. Pat. No. 3,849,156.
The primary advantages of heat curable mechanically frothed urethane foams stem from the decoupling of the foam generation step from the chemical curing step, as distinguished from processes which rely upon foam expansion during polymerization. The mechanically frothed process results in improved ability to control the product density and product dimensions of the final product compared to foams of the conventional method which rely on foam expansion during polymerization.
Molding of shaped urethane foam parts has been suggested in the art. With urethanes made by foam expansion during polymerization, a severe molding problem is encountered in the entrapment of air during mold filling which causes large voids in the molded part. This problem is especially serious when the mold cavities are small, contain channels of narrow cross section, or are of complex geometry. Many techniques are employed to eliminate voids in urethane casting, such as vacuum casting, centrifical casting, and compression molding. However, none of these techniques are applicable to the molding of parts with mechanically frothed urethane foams, because such foams are compressible and also because of a need to control the density of these materials.
The prior art also suggested the use of vents or ports to evacuate air or other gas from a mold. See, e.g., U.S. Pat. No. 1,910,526 to Dempsey and U.S. Pat. No. 2,550,140 to Dotson. In these patents, reliance is placed on the resin entering the mold cavity to force the gas from the cavity. However, these techniques are inadequate when molding shaped parts with mechanically frothed urethane foams because of problems that are peculiar to the molding of such froth foams. The prior art has also disclosed the process of filling the mold cavity from the bottom, such as in U.S. Pat. No. 4,046,942 to van Wersch. While this structure is effective in reducing some of the problems of air entrapment with some molding materials, air entrapment still remains a major problem when using mechanically frothed heat curable urethane materials as the molding material.
A persistent problem involves uneven filling of the mold cavity, which may also be accompanied by large voids or entrapped air 180.degree. from the sprue. This problem results from the fact that the mold cavity fills first in the vicinity of the sprue and fills last 180.degree. removed from the sprue. Thus, the mold cavity fills with an inclined flow front, and a substantial amount of material must flow out of the top of an open casting mold and be wasted before the entire mold becomes filled to form the desired part. A serious problem of the junction line or joining line (where the foam material meets in filling an annular mold) also persists when molding with mechanically frothed urethane foams. This junction line results from the meeting of but absence of lateral mixing of the foam in the mold at a location 180.degree. from the sprue.
With regard to junction lines or joining lines, a prior art patent of some interest is U.S. Pat. No. 1,918,532 to Guyer, where the stated purpose and the disclosed structure is to position the joining line at a preferred cite in the mold cavity. However, the junction line or joining line still exists.
While the ability to control product density makes it highly desirable to be able to mold parts from mechanically frothed urethane foams, the problems of air entrapment and the creation of a junction line or joining line are even more serious than with conventional urethane materials because of peculiarities of the mechanically frothed urethane foam material.